《复合材料 Composites》课程教学资源（学习资料）第五章 陶瓷基复合材料_j.1151-2916.1997.tb03063.x[1]

ournal . Am. Ceram Soc., 80 [7 1873-76(1997) In Situ Reacted Rare-Earth Hexaaluminate Interphases Markys G Cain, Rebecca L. Cain, t and Michael H. Lewis entre for Advanced Materials, University of Warwick, Coventry CV4 7AL, United Kingdom John Gent Rolls royce plc., Derby DE2 8BJ, United Kingdom A novel in situ reaction between a ceria-doped zirconia is taken up is dependent on the charge and radius of the cation interphase coating on Saphikon fibers and an outer alu- within the interspinal planes. A large number of rare-earth mina coating has resulted in the formation of oriented alkali, and alkaline-earth cations can stabilize the hexaalumi hexaaluminate platelets which can act as a low fracture nate structure in either its MP or B-alumina form. 3. 4 It is nergy interface barrier for crack deflection in oxide-oxide expected that the B-alumina and MP layered materials both ceramic- matrix composites( CMCs). The reaction proceeds only in reducing environments where the reduction of the of B-alumina, values of the fracture energy anisotropy, GG 3+ vale basal destabilization phenomenon consistent with previously re- enable their basal planes to deflect cracks since this fract ported findings. The diffusion of the cerium from the zir. energy ratio is <0. 25. The anisotropy in the growth kinetics conia into solid solution with the alumina can stabilize the of the hexaaluminates imparts a platelike or acicular morphol layered hexaaluminate structure. Preferred orientational ogy. This microstructural characteristic has been used as a growth of the hexaaluminate parallel to the coating inter- toughening agent in composites of alumina, zirconia, and alu- face was observed which is the required orientation for mina/zirconia containing the B/mP phase, processed in either a chanced debonding at the fiber/matrix interface in long dispersed form or formed in situ 16-21 The success of a refrac- fiber-reinforced CMCs tory layered-type hexaaluminate interphase would depend very much on the relative orientation of the B-alumina or MP basal planes with respect to the debonding interface- that is, the fiber surface in fiber-reinforced CMCs. Significant research has been conducted in this area and it remains uncertain wheth Lites(CMCs), the goal is to produce a material that will with- optimized such that the majority of the layered interphase pos sesses the required orientation. 6. 8, 22 This paper reports on a reason, much effort is devoted to oxide-oxide Cmc syste ly apyrores olenngle-crystal alumina fibers. An earlier study se- novel in situ formed hexaaluminate interphase material which imates the desired basal orientation with respect to the oxidation resistant 1-9 The success of an interface material for incorporation within oxide-oxide Cmcs depends largely upon lected rare-earth (rather than alkali)stabilized hexaaluminates tures. The former requirement can be attained through eith methods based on liquid precursors and magnetron sputter maintaining a low debond energy between the interface ing.8.9 Although matrix crack deflection was demonstrated by material and the fiber or matrix or through the materials microscopic observation of layer- plane separation in lanta own intrinsic, and necessarily low, fracture energy Materials num hexaaluminate crystals, there was limited preference for that possess a low intrinsic fracture enex lp (ucture alu- case saphikon monofilaments), resulting in high average C-axis orientation normal to the surface of the fibers (in this high-temperature stability include the rare-eart minates,8,9, which have a layered or sheetlike structure con- debond energies.23 There was also concern about filament taining weak basal planes which have been shown to readily cleave, 12 Hexaaluminates based on either the magnetoplum- strength degradation due to intrusion of the hexaaluminate bite or B-alumina structure have been proposed as suitable platelets into the filament surface during crystallization. An alternative approach which uses the fiber precoating as a interphase candidate materials II for use at temperatures urrently set by the silicate-reinforced Sic source of rare-earth stabilizing cation is described here. A for- tuitous in situ interface reaction has been observed when Ce. fiber composites. The magnetoplumbite (MP) and B-alumina doped zirconia/a alumina bilayer coated sapphire fibers are tructures are very similar, each having spinel blocks com- incorporated within an alumina matrix forming a hexaalumi sed of Ar+ and O?- ions with the same structure as spinel i3 nate interface. 8 The in situ formation of B or MP aluminas he differences lie in the contents and arrangement of the within oxide CMCs has been previously reported and shown to stabilizing cations between the spinel layers. The structure that occur in the presence of certain impurities (Mn, Mg, Sr ted the morph nd exact crystal structure of the hexaaluminate and, in tur the fracture toughness of the composite Preliminary investi D J. Green-contributing editor gations using composites containing Ce-doped zirconia/a alumina bilayer coated sapphire fibers were fabricated via the hot-pressing route to achieve acceptable densities. The process- ing atmosphere was considered of crucial importance in cata lyzing the in situ reaction. This paper reports on the evolution of the in situ reacted hexaaluminate interface in bilayer coated Saphikon sapphire fibers as a function of time and temperat

In Situ Reacted Rare-Earth Hexaaluminate lnterphases Markys G. Cain,* Rebecca L. Cain,' and Michael H. Lewis Centre for Advanced Materials, University of Warwick, Coventry CV4 7AL, United Kingdom John Gent Rolls Royce plc., Derby DE2 8BJ, United Kingdom A novel in sihc reaction between a ceria-doped zirconia interphase coating on Saphikon fibers and an outer alu￾mina coating has resulted in the formation of oriented hexaaluminate platelets which can act as a low fracture energy interface barrier for crack deflection in oxide-oxide ceramic-matrix composites (CMCs). The reaction proceeds only in reducing environments where the reduction of the cerium and zirconium ions to their 3+ valent state causes a destabilization phenomenon consistent with previously re￾ported findings. The diffusion of the cerium from the zir￾coda into solid solution with the alumina can stabilize the layered hexaaluminate structure. Preferred orientational growth of the hexaaluminate parallel to the coating inter￾face was observed which is the required orientation for enhanced debonding at the fibedmatrix interface in long￾fiber-reinforced CMCs. I. Introduction N THE next generation of advanced ceramic-matrix compos- I ites (CMCs), the goal is to produce a material that will with￾stand high temperatures in an oxidizing environment. For this reason, much effort is devoted to oxide-oxide CMC systems￾that is, oxide fibers and oxide matrices, which are inherently oxidation resistant.'-9 The success of an interface material for incorporation within oxide-oxide CMCs depends largely upon the interface material possessing the required debonding char￾acteristics'O and suitable chemical stability at service tempera￾tures. The former requirement can be attained through either maintaining a low debond energy between the interface material and the fiber or matrix or through the materials own intrinsic, and necessarily low, fracture energy. Materials that possess a low intrinsic fracture energy coupled with high-temperature stability include the rare-earth (RE) hexaalu￾minate~~.~.~." which have a layered or sheetlike structure con￾taining weak basal planes which have been shown to readily cleave6J2 Hexaaluminates based on either the magnetoplum￾bite or p-alumina structure have been proposed as suitable interphase candidate materials6.' I for use at temperatures higher than those currently set by the silicate-reinforced Sic fiber composites. The magnetoplumbite (MP) and p-alumina structures are very similar, each having spinel blocks com￾posed of A13+ and 02- ions with the same structure as pin el.'^ The differences lie in the contents and arrangement of the stabilizing cations between the spinel layers. The structure that D. J. Green-contributing editor yanuscript No. 191731. Received June 19. 1996 approved April 22, 1997. Member, American Ceramic Society. 'Now at Wmick Manufacturing Group, University of Warwick, Coventry, United Kingdom. is taken up is dependent on the charge and radius of the cation within the interspinel ~1anes.l~ A large number of rare-earth, alkali, and alkaline-earth cations can stabilize the hexaalumi￾nate structure in either its MP or p-alumina f~rm.'~.'~ It is expected that the p-alumina and MP layered materials both possess large fracture energy anisotropies. For single crystals of p-alumina, values of the fracture energy anisotropy, GJG - 0.01 l2 (Gb normal and Gp parallel to the basal plane) wouli enable their basal planes to deflect cracks since this fracture energy ratio is ~0.25.'~ The anisotropy in the growth kinetics of the hexaaluminates imparts a platelike or acicular morphol￾ogy. This microstructural characteristic has been used as a toughening agent in composites of alumina, zirconia, and alu￾mindzirconia containing the p/MP phase, processed in either a dispersed form or formed in situ.'6-21 The success of a refrac￾tory layered-type hexaaluminate interphase would depend very much on the relative orientation of the p-alumina or MP basal planes with respect to the debonding interface-that is, the fiber surface in fiber-reinforced CMCs. Significant research has been conducted in this area and it remains uncertain wheth￾er the processing and fiber coating route can be satisfactorily optimized such that the majority of the layered interphase pos￾sesses the required orientation.6*8*22 This paper reports on a novel in situ formed hexaaluminate interphase material which approximates the desired basal orientation with respect to the surfaces of single-crystal alumina fibers. An earlier study se￾lected rare-earth (rather than alkali) stabilized hexaaluminates as a potential interphase, on the basis of high-temperature sta￾bility, compatibility with oxides, and the identity of deposition methods based on liquid precursors and magnetron sputter￾ing.8.9 Although matrix crack deflection was demonstrated by microscopic observation of layer-plane separation in lantha￾num hexaaluminate crystals, there was limited preference for C-axis orientation normal to the surface of the fibers (in this case Saphikon monofilaments), resulting in high average debond energies.23 There was also concern about filament strength degradation due to intrusion of the hexaaluminate platelets into the filament surface during crystallization. An alternative approach which uses the fiber precoating as a source of rare-earth stabilizing cation is described here. A for￾tuitous in situ interface reaction has been observed when Ce￾doped zirconidcx-alumina bilayer coated sapphire fibers are incorporated within an alumina matrix forming a hexaalumi￾nate interface.8 The in situ formation of p or MP aluminas within oxide CMCs has been previously reported and shown to occur in the presence of certain impurities (Mn, Mg, Sr etc.)~J7.18*20.24 which also directly affected the morphology and exact crystal structure of the hexaaluminate and, in turn, the fracture toughness of the composite. Preliminary investi￾gations8 using composites containing Ce-doped zirconidcx￾alumina bilayer coated sapphire fibers were fabricated via the hot-pressing route to achieve acceptable densities. The process￾ing atmosphere was considered of crucial importance in cata￾lyzing the in situ reaction. This paper reports on the evolution of the in situ reacted hexaaluminate interface in bilayer coated Saphikon sapphire fibers as a function of time and temperature. 1873

of the ame The hot-pressing environment utilized above is typically re ducing but from experimental considerations it was also de of In reducing of oxygen), and at significantly lower temperatures, the phase assemblage is unstable and compounds such as Ce+AlO3 and I. Experimental Detai Ce2O3. 1lAl2O3(MP/B alumina) can form. Under reducing circumstances Ce4+ reduces to Ce+ by the reaction 2Ce02- the binary zirconia- kon sapphire fibers(Saphikon Inc, Milford, NH) by rf physical ceria phase diagram.2 Figures 2 and 3 show the difference in apor deposition(PVD)magnetron sputtering. Two PVD coat- constitut on for the bilayer coated fibers which have been sub- gs were deposited. The first layer in contact with the fiber jected to heat treatments at 1400C for 3 h in an argon atmo ere and 6 h in an air atmosphere, respectively. the grain additional set of coated fibers were supplied with only one reaction layers are clearly present in the fiber heat-treated in PVD ZrO /10% CeO2 coating so that the interfaces could be argon, one at the fiber surface and the other at the original studied as a function of single and combined layer chemistries interface een the two PVD layers. EDX analysis confirms Two or three short lengths of fiber from both series of coated fiber were heat-treated at either 1200 or 1400c for times B2 in Fig. 2, with compositions(charge balance uncorrected) varying between 10 min and 6 h in order to study the change CeAl o. 019.7 and CeAlz 1 O1s. 9, respectively, approximating in interfacial characteristics, viz., microstructure and composi Ce -/-MP alumina 3 TI tion with increasing time. In each instance the fibers were zirconia layer, labeled"ZrO2, "is depleted in cerium. No such heated(6C/min) from room temperature and cooled under phase instability was observed in coated fibers heat-treated in controlled conditions, using identical gas flow rates and alu- air under similar conditions(Fig. 3). This is consistent with mina crucibles. Fibers were sectioned and carbon coated prior some earlier work concerning Ce-stabilized zirconia. No to examination in the scanning electron microscope(SEM) other physical changes can be detected in the air-treated sample (EOL JSM-6100, Tokyo, Japan). All semiquantitative analysis and importantly no B/MP-alumina formation was observed as performed with the ultrathin window in position in the ighlighting the importance in composite fabrication X-ray detector(LINK/ISIS energy dispersive X-ray microanal ysis(EDX) system with 30 take-off detector) Divalent or other impurities were not found in any of the speci- mens during routine EDX analysis, suggesting that earlier re- shell will simulate the matrix in a composite and also form orted findings 6. 17, 18, 20, 24(discussed previously) could not ac- protective sheath for the"active"Ce-doped zirconia layer be count for the formation of the hexaaluminates or for the neath. The outer layer is very similar in composition to the destabilization of the Ce-(ss-Zro2 phase.(Identical ex matrix material but the residual stresses are anticipated to be ments were performed using single-coated fibers with no outer lifferent because of differing thermal histories. Both types of alumina shell. /MP-alumina was observed at the fiber/coatin fiber were heat-treated in air in addition to argon to give interface for all times at 1400oC but only at times exceeding 1 indication of the importance of the atmospheric conditions h in the 1200C experiments. The removal of the alumina shell does not appear to have inhibited the formation of hexaalumi nate Il. Results and discussion Work by heussner and Claussen and Zhu et al Figure l shows a fracture end of an as-received fiber. Both material of similar composition to the cerium-doped oatings on the fiber surface have a columnar grain structure used in this study, showed that in atmospheres of and EDX analysis correlated well with that expected from the original sputter tions. The coatings appear to be con tinuous with the columns of the outer alumina coating match ng those from the inner zirconia layer. 'Typical SEM EDX spot sizes were ar dimension to the crys In the ternary system CeOyZrO2/Al,O3 the binary phase resulting in only approximate analytical composit FIBR ZRO CE-ZRO2 ALUMINA Fig. 1. Bilayer PVD coated Saphikon fiber in theas-received Fig. 2. Bilayer PVD coated fiber heat-treated in argon at 1400oC for 6 h in argon

1874 Communications of the American Ceramic Society Vol. 80, No. 7 The hot-pressing environment utilized above is typically re￾ducing but from experimental considerations it was also de￾cided to perform a series of experiments in an inert (argon) atmosphere. 11. Experimental Details The oxide coatings were deposited onto single-crystal Saphi￾kon sapphire fibers (Saphikon Inc., Milford, NH) by rf physical vapor deposition (PVD) magnetron sputtering. Two PVD coat￾ings were deposited. The first layer in contact with the fiber was Ce (12%) doped zirconia and was -1.3 pm thick and the outer layer was alumina which was -3.5 km thick (Fig. 1). An additional set of coated fibers were supplied with only one PVD ZrO,/lO% CeO, coating so that the interfaces could be studied as a function of single and combined layer chemistries. Two or three short lengths of fiber from both series of coated fiber were heat-treated at either 1200” or 1400°C for times varying between 10 min and 6 h in order to study the change in interfacial characteristics, viz., microstructure and composi￾tion with increasing time. In each instance the fibers were heated (6”C/min) from room temperature and cooled under controlled conditions, using identical gas flow rates and alu￾mina crucibles. Fibers were sectioned and carbon coated prior to examination in the scanning electron microscope (SEM) (JEOL JSM-6100, Tokyo, Japan). All semiquantitative analysis was performed with the ultrathin window in position in the X-ray detector (LINKASIS energy dispersive X-ray microanal￾ysis (EDX) system with 30” take-off detector). Bilayer coated fibers are used such that the outer alumina shell will simulate the matrix in a composite and also form a protective sheath for the “active” Ce-doped zirconia layer be￾neath. The outer layer is very similar in composition to the matrix material but the residual stresses are anticipated to be different because of differing thermal histories. Both types of fiber were heat-treated in air in addition to argon to give an indication of the importance of the atmospheric conditions. 111. Results and Discussion Figure 1 shows a fracture end of an as-received fiber. Both coatings on the fiber surface have a columnar grain structure and EDX analysis correlated well with that expected from the original sputtered compositions. The coatings appear to be con￾tinuous with the columns of the outer alumina coating match￾ing those from the inner zirconia layer. In the ternary system CeO,/ZrO,/Al,O, the binary phase assemblage based on the ceridzirconia solid solution (ss) and alumina are reported stable at temperatures below 1600°C in air. In reducing environments, however (lower partial pressures of oxygen), and at significantly lower temperatures, the phase assemblage is unstable and compounds such as Ce3+A10, and Ce,O, 1 lAl,O, (MP/P alumina) can form. Under reducing circumstances Ce”+ reduces to Ce3+ by the reaction 2Ce0, + Ce203 + $,, which has implications for the binary zirconia￾ceria phase diagram.,’ Figures 2 and 3 show the difference in constitution for the bilayer coated fibers which have been sub￾jected to heat treatments at 1400°C for 3 h in an argon atmo￾sphere and 6 h in an air atmosphere, respectively. The grain structures of both fibers are observed to have coarsened but mo reaction layers are clearly present in the fiber heat-treated in argon, one at the fiber surface and the other at the original interface between the two PVD layers. EDX analysis confirms the presence of two in situ reacted interphases, labeled pl and p2 in Fig. 2, with compositions* (charge balance uncorrected) CeAl,,,,O,,., and CeAl,,,O,,,,, respectively, approximating the structural formula Ce,~,.Al12~y0,,~z-MP a1~mina.l~ The zirconia layer, labeled “ZrO,,” is depleted in cerium. No such phase instability was observed in coated fibers heat-treated in air under similar conditions (Fig. 3). This is consistent with some earlier work2, concerning Ce-stabilized zirconia. No other physical changes can be detected in the air-treated sample and importantly no P/MP-alumina formation was observed highlighting the importance in composite fabrication of using reducing or inert atmospheres to form the p/MP interphase. Divalent or other impurities were not found in any of the speci￾mens during routine EDX analysis, suggesting that earlier re￾ported finding^^*'^*'^,^^,*^ (discussed previously) could not ac￾count for the formation of the hexaaluminates or for the destabilization of the Ce-(ss)-ZrO, phase. (Identical experi￾ments were performed using single-coated fibers with no outer alumina shell. PW-alumina was observed at the fibedcoating interface for all times at 1400°C but only at times exceeding 1 h in the 1200°C experiments. The removal of the alumina shell does not appear to have inhibited the formation of hexaalumi￾nate.) Work by Heussner and Cla~ssen*~ and Zhu et aL2, using material of similar composition to the cerium-doped zirconia used in this study, showed that in atmospheres of low Po,, Typical SEM EDX spot sizes were of similar dimension to the crystallite sizes. However, beam broadening would sample a certain proportion of neighboring phases resulting in only approximate analytical compositions. Fig. 1. Bilayer PVD coated Saphikon fiber in the “as-received” state. 6 h in argon. Fig. 2. Bilayer PVD coated fiber heat-treated in argon at 1400°C for

Communications of the American Ceramic Society a Ym 20 FIBRE ZRO 3. Bilayer PVD coated fiber heat-treated in air at 1400oC for Fig. 4. Bilayer PVD coated fiber heat-treated in argon at 1200C for 3 h. Note the presence of only one hexaaluminate reaction interphase reduction of the Ce4++, Ce3+ a phase separation of the mobile cation, Ce+, from the destabilized zirconia into the the tetragonal()Ce-stabilized to monoclinic zirco- outer alumina phase. Alternatively if the hexaaluminate wa a (on cooling below Tt-m)+ as predicted by the phase diagra m destabilization alumina then the continual depletion of the p phase would is related to the corresponding increase in ic adii of the control the reaction This would also account for the lack of cerium cation Ce3+(1. 07 A)compared to Ce++(0.94 A)leading evidence for the pyrochlore phase during routine examination o a decrease in solubility of CeO, in the tetragonal Zro2. This in the sEM. There is presently insufficient data, however, to results in the destabilization of the t phase in reducing envi- accurately describe the reaction kinetics. Further detailed ronments. Zhu et al.26 also noted that no such phase separation analysis of the evolution of the interface using higher resolu transformation occurred in air or in reducing atmospheres tion microscopy (tEm)is clearly required to be able to resolve ow 1000C. Although the cerium was reduced under these these claims conditions, it was additionally noted by these authors that The observation of the morphologically distinct basal planes hase separation/transformation only occurred at a critical tem of the hexaaluminate allel to the Zro/alo it perature of over 1200C when Zr++ was also reduced to Zr+. terface implied in Figs. 4 and 2 is encouraging in view of the The reducing atmosphere was hydrogen, which is severe known debonding requirements in CMCs. This special orien- enough to reduce the zirconium from its pentavalent to its tation exhibited by the MP layers most likely arises from the trivalent state. The reduction of the zirconia may not, however growth anisotropy of the be a prerequisite for the destabilization phenomenon where hexaaluminates will grow with their basal planes parallel to the there is a sink for the reduced Ce3+ -the alumina shell and/or reaction direction because the rapid transport paths along the fiber in the present case. In other studies concerning the effects basal planes are aligned with the transport direction, Alterna- of various atmospheres on the phase relations in the system tively, it is commonly the free surface energy that determines Zr-Ce-0, 28,29 the Ce4+ in solid solution with the Zro2 is re the growth habit of"single crystal"thin films. In the case of duced to Ce3+ at increased temperatures in reducing environ- pure alumina, the free surface energy of the fully relaxed basal Xrd data from such small specimens and the pyrochlore phase related structures such as B/MP-dunee mip mplicaaincrys. ments(CO, H, NH,,), in vacuums(10- to 10-2 Pa), and in inert planes(0001)is significantly lower than most other gen partial pressures. In this study it was not possible to gate Atmospheres(Ar, He)as well as in atmospheres with low on allographic directions. 30 This result has clear and, possibly, was not observed using EDX analysis in the SEM(but see Of the samples investigated, none showed full conversion of t possible to determine if the outer alumina shell to hexaaluminate. It is anticipated that ffusion of the free Ce+ ion or the reaction of alumina with the formation of the hexaaluminate layer is controlled by the the compound Zr2 Ce20, forms the hexaaluminate observed at diffusion of oxygen to the surface of the coated fiber along an activity gradient which decreases as the surface is approached There are two competing reaction fronts during heat treat In porous materials, such as these, argon/oxygen diffusion may ment, one at the fiber surface and one at the Pvd alumina be channeled along pores and through the columnar structure of shell/PVD zirconia interface. In all the specimens examined, he Pvd coatings, thus increasing the yield of reduced ce+at ome evidence of hexaaluminate formation was detected. Fi- the outermost interface resulting in a tendency for this layer to bers heat-treated at 1200 C exhibited a single hexaaluminate grow preferentially( Fig. 5). It is thus probable that tailoring of reaction layer situated between the two PVd layers( Fig 4).At the hexaaluminate interface may be achieved by variation of this stage no hexaaluminate was observed at the zirconia/fiber deposition parameters, i.e., controlling coating porosity and interface. It was also observed that very little grain growth had structure. The schematic in Fig. 5 represents the gradient in the taken place in the outer layer but the zirconia layer is signifi- postulated valence state of the Ce ion which is initially in state cantly more granular. EDX analysis yielded significant levels Ce and in solid solution with the zirconia In reducing envi- f Ce(1.5 at. %)retained within the zirconia at 1200C. At ronments the valency changes to its Ce+ state(and at suffi 1400 C hexaaluminate was readily observed at both interfaces ciently high temperatures, >1200.C, Zr+-Zr+ also)which Fig. 2). The evolution of the hexaaluminate layers(Figs. 2 and s no longer able to stabilize the tetragonal form of the zirconia. 4)in terms of thickness, extent, and composition was indicative The destabilization and phase partitioning that occurs can pro- a diffusion-rate-controlled reaction where the reaction prod mote the formation of these in situ reacted hexaaluminate lay uct, in this case hexaaluminate, inhibits the further diffusion of ers. Alternatively, the reduced valent Ce3+ ions, no longer able

July 1997 Communications of the American Ceramic Society 1875 Fig. 3. Bilayer PVD coated fiber heat-treated in air at 1400°C for 3 h. reduction of the Ce4+ + Ce3+ resulted in a phase separation of the tetragonal (t) Ce-stabilized zirconia into monoclinic zirco￾nia (on cooling below T,-,,J + pyrochlore (P) phase Zr,Ce207 as predicted by the phase diagram.25 The t + m destabilization is related to the corresponding increase in ionic radii of the cerium cation Ce3+ (1.07 A) compared to Ce4+ (0.94 A) leading to a decrease in solubility of CeO, in the tetragonal Zro,. This results in the destabilization of the t phase in reducing envi￾ronments. Zhu er a1.26 also noted that no such phase separation and transformation occurred in air or in reducing atmospheres below 1OOO"C. Although the cerium was reduced under these conditions, it was additionally noted by these authors that phase separatiodtransformation only occurred at a critical tem￾perature of over 1200°C when u" was also reduced to Zs+. The reducing atmosphere was hydrogen, which is severe enough to reduce the zirconium from its pentavalent to its trivalent state. The reduction of the zirconia may not, however, be a prerequisite for the destabilization phenomenon where there is a sink for the reduced Ce3+-the alumina shell andor fiber in the present case. In other studies concerning the effects of various atmospheres on the phase relations in the system Zr-Ce-0,28*29 the Ce& in solid solution with the Zr02 is re￾duced to Ce3+ at increased temperatures in reducing environ￾ments (CO, H, NH,), in vacuums (lo-' to lo-, Pa), and in inert atmospheres (Ar, He) as well as in atmospheres with low oxy￾gen partial pressures. In this study it was not possible to gather XRD data from such small specimens and the pyrochlore phase was not observed using EDX analysis in the SEM (but see previous footnote). It was thus not possible to determine if diffusion of the free Ce3+ ion or the reaction of alumina with the compound Zr2Ce,07 forms the hexaaluminate observed at the interfaces. There are two competing reaction fronts during heat treat￾ment, one at the fiber surface and one at the PVD alumina shell/PVD zirconia interface. In all the specimens examined, some evidence of hexaaluminate formation was detected. Fi￾bers heat-treated at 1200°C exhibited a single hexaaluminate reaction layer situated between the two PVD layers (Fig. 4). At this stage no hexaaluminate was observed at the zirconidfiber interface. It was also observed that very little grain growth had taken place in the outer layer but the zirconia layer is signifi￾cantly more granular. EDX analysis yielded significant levels of Ce (-1.5 at.%) retained within the zirconia at 1200°C. At 1400°C hexaaluminate was readily observed at both interfaces (Fig. 2). The evolution of the hexaaluminate layers (Figs. 2 and 4) in terms of thickness, extent, and composition was indicative of a diffusion-rate-controlled reaction where the reaction prod￾uct, in this case hexaaluminate, inhibits the further diffusion of Fig. 4. Bilayer PVD coated fiber heat-treated in argon at 1200°C for 3 h. Note the presence of only one hexaaluminate reaction interphase. the mobile cation, Ce3+, from the destabilized zirconia into the outer alumina phase. Alternatively if the hexaaluminate was formed via a reaction of the P phase-Zr,Ce,O,-with the alumina then the continual depletion of the P phase would control the reaction. This would also account for the lack of evidence for the pyrochlore phase during routine examination in the SEM. There is presently insufficient data, however, to accurately describe the reaction kinetics. Further detailed analysis of the evolution of the interface using higher resolu￾tion microscopy (TEM) is clearly required to be able to resolve these claims. The observation of the morphologically distinct basal planes of the hexaaluminate growing parallel to the Zr02/A1203 in￾terface implied in Figs. 4 and 2 is encouraging in view of the known debonding requirements in CMCs. This special orien￾tation exhibited by the MP layers most likely arises from the growth anisotropy of these phases, discussed earlier. Often hexaaluminates will grow with their basal planes parallel to the reaction direction because the rapid transport paths along the basal planes are aligned with the transport direction. Alterna￾tively, it is commonly the free surface energy that determines the growth habit of "single crystal" thin films. In the case of pure alumina, the free surface energy of the fully relaxed basal planes (OOO1) is significantly lower than most other main crys￾tallographic directions.30 This result has clear implications on the preferred growth orientations of alumina and, possibly, related structures such as P/MP-alumina. Of the samples investigated, none showed full conversion of the outer alumina shell to hexaaluminate. It is anticipated that the formation of the hexaaluminate layer is controlled by the diffusion of oxygen to the surface of the coated fiber along an activity gradient which decreases as the surface is approached. In porous materials, such as these, argodoxygen diffusion may be channeled along pores and through the columnar structure of the PVD coatings, thus increasing the yield of reduced Ce3+ at the outermost interface resulting in a tendency for this layer to grow preferentially (Fig. 5). It is thus probable that tailoring of the hexaaluminate interface may be achieved by variation of deposition parameters, i.e., controlling coating porosity and structure. The schematic in Fig. 5 represents the gradient in the postulated valence state of the Ce ion which is initially in state Ce4+ and in solid solution with the zirconia. In reducing envi￾ronments the valency changes to its Ce3+ state (and at suffi￾ciently high temperatures, >1200"C, Uk + W+ also) which is no longer able to stabilize the tetragonal form of the zirconia. The destabilization and phase partitioning that occurs can pro￾mote the formation of these in situ reacted hexaaluminate lay￾ers. Alternatively, the reduced valent Ce3+ ions, no longer able

1876 Communications of the American Ceramic Society Vol. 80, No. 7 2A. G. Evans. f. W. Zok Davis,“ The Role of I P/B es, "Compos. Sci K、Ce Mater, Res, Soc. Symp. Proc., 20, 697-712(1984) 4A. Kelly, Design of a Possible Microstructure for High Temperature Ser MP B vice, 'Ceram. Trans., 57, 117-29(1995 SR Lundberg and L. Eckerbom, Design and Processing of All-Oxide Com- posites, " Ceram. Trans., 58, 95-104(1995) Oxide Composites, "Mater Sci. Eng, A162, 15(1993) J. Whalen. D Narasimhan, C. G. Gasdaska, w. ODell, and R C. morris, G. Razzell, and 3, Gent, ""Devel f Interfaces in Oxide and Silicate-Matrix Composites, Ceram, Trans. 58,41-52(1995 loA. G. Evans and D. B. Marsha Mechanical Behaviour of Ceramic hexaaluminate Matrix Composites, Acta Metall. 37 [10] 2567-83(1989) Fig. 5. Schematic showing preferential growth of outer hexaalumi nate laye 12D. C. Hitchcock and L C De Jonghe, ""Fracture Toughness Anisotropy of Sodium-B-Alumina, J, Am. Ceram Soc., 66 19]1053-67(1983) N. lyi, S. Takekawa, and S. Kimura, "The Crystal Chemistry of Hexaal tes: B-Alumina and Magnetoplumbite Structures, "J. Solid State Chem to remain in solid solution with the zirconia, directly stabilize 83, 8-19(1989) the hexaaluminate through solid solution reaction with the alu- P.E. D. Morgan and J. A Miles, ""M: mina. The gradient in the oxygen partial pressure is responsible Further Discussion J, A Ceram Soc., 69 the zirconia coating. In reducing conditions, the defect center in tween Dissimilar Materials, "Int J. Solids Struct, 25, 1053-67(1989/e be. SM. Y. He and J w. Hutchinson ' Crack Deflection at the Interf Ce-stabilized zirconia is an oxygen ion vacancy which com M. Miura, H. Hongoh, T. Yogo, S. Hiran T Fuji, Formation of es for the reduced valent Ce3+ and can be La-B-Aluminate Crystal in Ce-TZP, J, Mater. Sci., 29, 262-c 2Ce+++0o 2Ce3++v6+102T with V representing an (1994) oxygen vacancy and Oo representing an oxygen on an oxygen 17P.-L Chen and I.-w. Chen, "" In-situ Alumina/Aluminate Platelet Compos- ites, 'J Am. Ceram Soc., 75 [9]2610-12(1992) site. Clearly, in atmospheres of excess oxygen the reaction is buffered and proceeds in the opposite direction thereby stabi- Plasticity and Toughening in CeO-Partially-Stabilized Zirconia-Alumina(Ce and others, even slightly reducing conditions can result in the 38(1992) loss of oxygen from the system leading to the stabilization of r9R. A. Cutler, R J. Mayhew, K M. Prettyman, and A. v. virkar stable only in oxidizing atmospheres. The stability of the RE \Ww1/S Ce-TZP the 3+ valent state. It thus appears that Ce-stabilized zirconia is B/MP-aluminas in oxide site systems has been demon- 2H- D. Kim, L-S. Lee, S.-w. Kang, and J.w. Ko, The Formation of strated in high-temperature oxidizing heat treatment studies up ties of Alumina, "J. Mater. Sci, 29, 4119-24(1994) Mechanical Proper- to 1500C, against alumina and unstabilized zirconia inter 2T. B. Troczynki and P. S. Nicholson, Resistance to Fracture of a Partia faces.8. 31 Stabilized Zirconia/B-Alumina Composite, J. Am. Ceram Soc., 68[10]123- 35(1985) M. K. Cinibulk and R S Hay, Textured I. Conclusions Ite Fiber-Matrix The reduction of Ce++to Ce+(and possibly Zr+to Zr+)has 1233-46(1996 been shown to result in the development of in situ reacted ture and Stability of Synthetic Interphases in CMCs, "Key Eng. Mater, 127 hexaaluminate interphases. The reaction occurs via one of two l3I[Part137-50(1997 routes:(1)A Ce solute diffusion from the tetragonal solid solution and phase partitioning into an unstabilized zirconia has not been observed in these in situ reacted studies, it is from 1350 c to ig and T. Sata, "Phase Studies in the Sy Zr02 -Ce2 O3 C, Bull. Tokyo Inst Tech 8,25-32(1972) expected that such a phase would react with alumina, forming 2H. -Y, Zhu, T. Hirata, and Y Muramatsu, Phase Separation in 12 mol% zirconia and hexaaluminate. (2)The destabilization of the Ce- Soc, 75 (10 2843-48(1992) Induced by Heat Treatment in H, and Ar, J. Am Ceran luminate via directed diffusion along grain boundaries and/or onal Zirconia polycrystals by reduc the columnar structure of the PVd coatings through a REDOX Am. Ceram Soc. 72(6)1044 46(1902nduced Phase Trans type reaction. Preferred orientational growth of the hexaalumi ZRA. L Leo A. B. Andreeva, and E.K. Keler. Influence of the Gas nate parallel to the coating interface was interpreted from its Atmos of Zirconium dioxide with Oxides of Cer morphological character which is in the correct habit for en- [zv. Akad. Nauk SSSR, Neorg. Mater. 2 [1] 137-44(1966) hanced micro/macromechanical properties. nd E. K. Keler. *I hemical Reactions and Polymorphic Transfor tem Zirconium Dioxide-Cerium Oxides, Ogneupory, 31 [3] 42-48 301. Manassidis and M. J. Gillan, ""Structure and Energetics of nd P c. hall faces Calculated from First Principles, " J. Am. Ceram. Soc., 77[2]335-38 with Ap m.Soc,765]1265-73(1993) 3IM. G. Cain; unpublished work, 1995

1876 Communications of the American Ceramic Society Vol. 80, No. 7 Fig. 5. Schematic showing preferential growth of outer hexaalumi￾nate layer. to remain in solid solution with the zirconia, directly stabilize the hexaaluminate through solid solution reaction with the alu￾mina. The gradient in the oxygen partial pressure is responsible for setting up the gradient in the Ce3+/Ce4+ ratio which spans the zirconia coating. In reducing conditions, the defect center in Ce-stabilized zirconia is an oxygen ion vacancy which com￾pensates for the reduced valent Ce3+ and can be written as 2ce4+ + o0 4 2ce3+ + G- + 1/20,T with 6- representing an oxygen vacancy and Oo representing an oxygen on an oxygen site. Clearly, in atmospheres of excess oxygen the reaction is buffered and proceeds in the opposite direction thereby stabi￾lizing the 4+ valent state. However, from these experiments and others, even slightly reducing conditions can result in the loss of oxygen from the system leading to the stabilization of the 3+ valent state. It thus appears that Ce-stabilized zirconia is stable only in oxidizing atmospheres. The stability of the RE P/MF’-aluminas in oxide composite systems has been demon￾strated in high-temperature oxidizing heat treatment studies up to 150O0C, against alumina and unstabilized zirconia inter￾face~.~”’ IV. Conclusions The reduction of Ce4+ to Ce3+ (and possibly ZP+ to 213’) has been shown to result in the development of in siru reacted hexaaluminate interphases. The reaction occurs via one of two routes: (1) A Ce solute diffusion from the tetragonal solid solution and phase partitioning into an unstabilized zirconia and a pyrochlore, P, phase Zr,Ce,O,. Although this P phase has not been observed in these in situ reacted studies, it is expected that such a phase would react with alumina, forming zirconia and hexaaluminate. (2) The destabilization of the Ce￾Zro, would liberate Ce3+ to immediately stabilize the hexaa￾luminate via directed diffusion along grain boundaries and/or the columnar structure of the PVD coatings through a REDOX type reaction. Preferred orientational growth of the hexaalumi￾nate parallel to the coating interface was interpreted from its morphological character which is in the correct habit for en￾hanced micro/macromechanical properties. References ‘R. F. Cooper and P. C. Hall, “Reactions between Synthetic Mica and Simple Oxide Compounds with Application to Oxidation Resistant Ceramic Compos￾ites,” J. Am. Ceram. Soc., 76 [5] 1265-73 (1993). ,A. G. Evans, F. W. Zok, and J. Davis, “The Role of Interfaces in Fiber￾Reinforced Brittle Matrix Composites,” Compos. Sci. Technol., 42, 3-24 (1991). 3W. B. 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